How to use Red/System code from Red?

This is frequently asked question, so I decided to write a simple guide. It will take about 5 minutes to read it AND test it. Yes, it’s that easy.

First thing you need to know is that using Red/System code requires the Red code to be compiled. It is not possible to use R/S code in interpreter. At least yet. So, how to do it? There are more ways, let’s look at them.

#system

#system directive lets you use Red/System code in Red directly. Here's an example:

Red[]
print "This print is on Red level and therefore adds newline."
#system [
print "This print is on R/S level, so no newline here."
]
print "See? Now we are back in Red."

routine

Using #system is nice, but you may want to reuse the code. Red provides you with routine function that is similar to func, but its body is written in Red/System instead of Red. See example:

#include

You may also want to use existing R/S code or split your source to Red and Red/System part. For adding source file, we use #include directive in Red, but for Red/System code we need to use #include on R/S level. So instead of #include %my-rs-file.reds we need to use #system [#include %my-rs-file.reds]. Here is an example:

Red/System part

First we need some Red/System code. Copy this source and save it as %test.reds:

Red part

As you can see, here we are using both #system directive and routine to use Red/System in Red.

Another thing that the last example introduced is passing arguments from Red to R/S code and vice versa. Please note that this is possible directly for scalar values like integer! or float!, but for datatypes like string! or block!, some conversion must be done on Red/System level. IT IS NOT POSSIBLE TO USE THESE TYPES DIRECTLY IN RED/SYSTEM! The conversion is not that hard but that is something outside the scope of this article. So, maybe next time.